The present invention relates generally to a system and method for controlling a fuel vapor recovery system in an internal combustion engine.
Motor vehicles of recent years typically incorporate a fuel vapor recovery system in connection with the internal combustion engine to reduce the amount of fuel vapors released into the atmosphere from the fuel tank. Typically, a canister containing a fuel vapor absorbing material, such as activated charcoal, is coupled between the fuel tank and the air/fuel intake of the engine. A purge valve positioned between the canister and air/fuel intake of the engine facilitates the periodic purging of fuel vapors from the canister. The fuel vapor absorbing material absorbs fuel vapor from the fuel tank. The purged fuel vapors are channeled into the air/fuel intake of the engine.
Generally, it is desirable to maximize the amount of fuel vapor purged from the canister over a given period of time to continuously maintain adequate storage capacity of the canister. However, the addition of fuel vapor to the air/fuel intake increases the amount of fuel supplied to the engine cylinders, thereby altering the engine air/fuel ratio. The desired engine air/fuel ratio is carefully determined to ensure favorable drivability characteristics and to control undesirable engine emissions. To avoid altering the desired air/fuel ratio, and thus avoid undesirable engine emissions and adverse vehicle drivability (such as transient torque fluctuations), the amount of fuel vapor added to the air/fuel intake should be controlled so that certain engine operating parameters are satisfied.
Known systems and methods for purging the vapor recovery system attempt to achieve the above-described objectives by gradually opening the purge valve over a period of time. As the purge valve is slowly opened, an electronic engine controller determines, or xe2x80x9clearnsxe2x80x9d, the amount of fuel vapor that flows through the purge valve and into the engine air/fuel intake. The controller also observes various engine operating parameters, such as engine air mass, fuel injector pulse width, and fuel tank sloshing conditions, to determine if the level of fuel contribution currently provided from the vapor recovery is likely to cause transient torque fluctuations or otherwise degrade vehicle drivability. Such operating parameters are interchangeably referred to herein as xe2x80x9csystem constraints.xe2x80x9d If all of the operating parameters, or system constraints, are within acceptable ranges, the purge valve continues to be gradually opened. On the other hand, if one or more of the operating parameters falls out of acceptable range, the purge valve is completely closed, and the process of gradually opening the purge valve begins again.
The inventors herein have recognized that when utilizing the known method, the purge valve is repeatedly closed during engine operation due to one of the engine operating parameters falling out of an acceptable range. Further, the inventors have recognized that the greatest amount of uncertainty concerning the amount of fuel vapor being purged occurs during the initial opening of the purge valve. Thus, because the purge valve is being repeatedly closed, the calculated fuel vapor amount being supplied from the vapor recovery system may have a relatively large error whenever the purge valve is reopened. Thus, a total calculated fueling amount representing the sum of the desired fuel amount from the fuel injectors and the calculated fuel vapor amount may also have an error immediately after the purge valve is reopened. The error in the total calculated fuel amount may result in undesirable transient torque fluctuations, decreased fuel economy, and degraded emission control.
A new system and method is provided for controlling the amount of fuel vapors delivered to the engine cylinders from the fuel vapor recovery system. In connection therewith, a new system and method is provided for evaluating various engine operating parameters that are indicative of vehicle drivability characteristics and for determining which of the parameters is the most limiting constraint on the fuel vapor recovery system. More specifically, the current values of various operating parameters are each compared to a respective maximum value or reference value to determine a unique xe2x80x9cpurge flow ratioxe2x80x9d associated with each operating parameter. Each purge flow ratio is indicative of the difference between the current value of the operating parameter and either the desired value or maximum value for that same operating parameter. The engine operating parameter associated with the minimum purge flow ratio is considered to be the overall most limiting system constraint. Thus, the purge flow of the vapor recovery system is adjusted based on the minimum purge flow ratio.
If the minimum purge flow ratio is greater than a reference value, preferably 1.0, then the purge flow from the vapor recovery system can be safely increased. The minimum purge flow ratio may be used to determine an appropriate amount to increase the purge flow. On the other hand, if the minimum purge flow ratio is less than the reference value, then the purge flow from the vapor recovery system should be decreased. Instead of completely closing the purge valve, as in the prior art, the purge valve is only partially closed, the degree to which may be determined based on the value of the minimum purge flow ratio.